In electric vehicles, such as a hybrid or electric car, regenerative energy is stored into a battery via a power converter including an inverter. This regenerative power can be reused to increase the effectiveness of the vehicle. However, during regenerative control, when a problem, such as a breakage in a cable connecting the battery to the power conversion device, arises so that the battery and the power conversion device come into a disconnection state, the regenerative power cannot be stored into the battery. For example, when this breakage occurs between the battery and a smoothing capacitor for the battery, all the regenerative power other than power consumed by the parasitic resistance of a wire and the ON resistance of each switching element of the inverter, is stored into the smoothing capacitor. When the regenerative power continues to be stored into the smoothing capacitor, the voltage of the smoothing capacitor increases to be an overvoltage, which decreases the reliability of the smoothing capacitor and each switching element of the inverter.
A conventional inverter stops a motor by turning on all lower switching elements, connected to the negative side of a DC power supply, of pairs of switching elements for respective phases of the motor. Accordingly, the respective phases of the motor are short-circuited to each other, and energy that has driven the motor so far is electrically consumed by a load such as a coil of the motor (see, for example, Patent Document 1).
A conventional power conversion device is disclosed which inhibits an increase in voltage of a DC power supply line during regeneration operation by the following method.
When an overvoltage of the DC power supply line is detected, a three-phase short-circuit is caused to occur. Because of the three-phase short-circuit, a current flows back between a motor and semiconductor switching elements, so that an increase in voltage of the DC power supply line can be inhibited. In the three-phase short-circuit, semiconductor switching elements of an upper arm or semiconductor switching elements of a lower arm are turned on for causing the current to flow back. Thus, the semiconductor switching elements that are turned on generate heat. In order to prevent occurrence of a breakdown of the semiconductor switching elements due to heat, the arm in which the three-phase short-circuit occurs is switched as appropriate. The arm in which the three-phase short-circuit is caused to occur may be switched on the basis of outputs of temperature sensors, which are provided at the upper arm side and the lower arm side of an inverter portion, respectively, such that the temperature of the semiconductor switching elements at the upper arm side are substantially equal to that at the lower arm side (see, for example, Patent Document 2).